The illumination of a large area by a central lighting system has long been desired because of its many advantages. For example, a centralized light source is easier to maintain than many distributed light sources. Heat generated by a centralized light source can be easily vented from the lighted areas, whereas heat from distributed light sources is not easily vented. A light distribution system connected to a centralized light source radiates minimal or no heat.
Light pipes or light conduits made of a transparent material having substantially planar inner surfaces and outer surfaces which are "in octature" have been utilized to transport light, as illustrated in U.S. Pat. No. 4,260,220 to Whitehead. These devices are typically constructed of an optical lighting film made of flexible polymeric sheets of a transparent material having a structured surface on one side and a smooth surface opposite the structured surface. The structured surface of the devices preferably includes a linear array of miniature substantially right angle isosceles prisms arranged side-by-side to form a plurality of peaks and grooves. Further, the perpendicular sides of the prisms make an angle of approximately 45 degrees with the smoother surface. This structure of the polymeric sheets, as well as the shape of the light conduit, enables light to be constrained to travel through the light conduit without escaping through its walls if the angle by which the light rays deviate from the longitudinal axis of the light conduit does not exceed a critical angle. Thus, light entering a light conduit at an angle less than the critical angle is totally internally reflected.
The critical angle is defined as the arc sine of the ratio of the index of refraction of the surrounding medium (typically air) to that of the wall material. For example, for a transparent material of polymethylmethacrylate having a refractive index of 1.493, all incident light rays less than the critical angle of about 27.3 degree, as measured along the longitudinal axis of the light conduit, will be totally internally reflected. On the other hand, incident light that enters the light conduit outside the critical angle will not be totally internally reflected.
Alternatively, a light conduit can be constructed using a multi-layer optical film, such as disclosed in U.S. Pat. No. 5,661, 839 (Whitehead). Light conduits have been constructed with various cross-sections, such as square cross-sections as illustrated in U.S. Pat. No. 4,260,220, and circular cross-sections, as illustrated in U.S. Pat. No. 4,805,984.
In many applications, it is desirable to allow the light to escape from the light conduit in a controlled manner. Many means for facilitating emission of light from the light conduit have been used in the past, such as disclosed in U.S. Pat. No. 5,363,470 (Wortman). In another example, an extractor such as a diffuse scatterer made of a highly reflective white polymeric tape such as SCOTCHCAL ELECTROCUT brand film, manufactured by Minnesota Mining and Manufacturing Company, St. Paul, Minn., can be placed inside a light conduit to increase the rate of leakage, or emission, of the light from inside the light conduit. The diffuse scatterer increases the rate of leakage by "scattering" light that hits it into non-total internal reflecting angular regions of the light conduit, thereby increasing the amount of light in those angles which allow light to be emitted from the light conduit. Typically, a strip of the highly reflective while polymeric tape is placed over the length of the light conduit to cause the scattering.
Current light distribution systems utilize segmented light conduits attached to a light source. The light conduit segments are joined to form a chain which can have an overall length between 4 and 40 meters. A light distribution system can include a single light source provided at one end of the chain or include light sources at each end, which are directed toward each other. Light distribution systems are subjected to various forces including tension and compression forces produced by thermal contraction and expansion, respectively, which accompany temperature fluctuations. A 40-meter light conduit chain can expand and contract by as much as 15 centimeters. Accordingly, light distribution systems must be designed to withstand the forces produced by such changes in length. In current installations, the light source is typically attached to a wall or ceiling at a fixed location. As such, the light source cannot move to relieve stresses in the light distribution system and the location cannot be easily changed after installation. In addition, for a two-light system in which one of the light sources must be permitted to move relative to the other, installation requires two separate light source mounting assemblies. It would therefore be desirable to provide a mounting assembly for light sources which eliminates the need for separate mounting assemblies, and which is easy to install.